The invention relates to an air spring mounting system for truck cabs, and more specifically to the means of fin tuning the spring rate, damping, and load capacity of the air springs used to resiliently mount the cab onto the chassis of a truck.
Trucks have for a long time been considered to be unacceptable in terms of vibration levels in the cab. There have been two areas for concern here, the comfort and health of the driver, and increased maintenance costs caused by high vibration levels. The source of the vibration is not so much road induced shock as it is free vibration of the frame excited by wheel imbalance and other driving forces.
Present day trucks are necessarily fitted with quite flexible frames, and the result is that, the frame has many modes between 3 and 12 Hertz. This not only causes high vibration levels in the cab, but also complicates the job of isolating the cab. To isolate the vibration, the suspended mass system must have a natural frequency, which is less than the lowest natural frequency of the frame divided by 2 times the square root of 2. This the lowest natural frequency of the frame. This requires very low spring rates, and the resultant large sway space allowance is not acceptable for most installations. A large sway space is not needed for most vibrations, but must be allowed for in case hard bumps are encountered. And most air springs have insufficient volume to achieve the needed low frequencies.
Heretofore designers have used only approximately the correct volume of air, and corrected for this with the use of multiple strong hydraulic shock absorbers to control sway during resonance, which may be driven by a near by frame mode. This then has a detrimental effect on the quality of ride achieved and on the life of the shock absorber.
Another design has used long thin air springs in a effort to increase the air volume in the spring. This has led to unstable springs which collapse frequently.
Additionally, the space available for installation of the air spring suspension system is generally quite restricted. This makes a small air spring desirable, but small springs lack the necessary load capacity and air volume. Some previous designs use two springs, but then the desirable three point mounting arrangement is defeated, and the spring rate is increased to the detriment of the isolation.